Vacuum contact printing for the graphic arts is well known and wherein upon a bed there is normally mounted in a horizontal position a blanket frame which includes a flexible impervious blanket having a peripheral bead. A glass frame is positioned above the blanket frame and adapted for snug registry with its bead for defining a contact printing chamber between the glass frame and blanket. Hinges or other linkage interconnect the glass frame with the blanket frame so that the glass frame may be pivoted or otherwise elevated with respect to the blanket frame in order to permit loading upon the blanket of a light sensitive sheet and thereover a negative. A vacuum source is provided having a conduit for providing vacuum communication to the printing chamber. After a predetermined period the flexible blanket of the blanket frame is drawn tightly and snugly up against the glass of the glass frame after which a light is presented to the frame assembly either in a horizontal or vertical position for exposing the sensitized sheet or plate. Examples of that type of vacuum printer are shown in the following United States prior art patents:
______________________________________ U.S. PAT. NO. DATE NAME ______________________________________ 2,287,228 June 23, 1942 J. T. Cambell 2,376,416 May 22, 1945 J. T. Cambell 2,494,282 January 10, 1950 J. T. Cambell 2,270,578 January 20, 1942 J. T. Cambell 3,106,145 October 8, 1963 L. Hutchins ______________________________________
Most of the foregoing patents show the relationship between a flexible blanket within a blanket frame and the glass of the overlying glass frame wherein the chamber between the two frames, sometimes referred to as a vacuum printing chamber, is evacuated so as to establish a good contact between the negative and the sensitized sheet and the glass frame at the time that it is exposed by the momentary application of the source of light.
The problem has heretofore existed that though every effort is made to completely evacuate the vacuum chamber to establish a snug contact between the flexible blanket and the glass of the glass frame, it appears that though not intended in many cases, the initial application of vacuum to the vacuum chamber moves such portions of the blanket with respect to the glass of the glass frame that air is actually entrapped within the vacuum chamber, preventing perfect contact. This occurred because the vacuum started at the perimeter of the frames and moved toward the center.
This is productive of exposed negatives which are not one hundred percent effective due to a lack of a perfect contact between the glass and blanket and the negative and sensitized sheet therebetween.
In the prior art patents are practiced considerable difficulty has been encountered in attempting to obtain a complete evacuation of all of the air between the glass and blanket frames hopefully to obtain perfect exposure photographically on to the sensitized sheet. Efforts have been made to provide a more complete evacuation of the vacuum printing chamber.
In the copending application provision is made for a vacuum printer which obtained almost perfect and fast evacuation of air from the printing chamber. This was accomplished by an initial application of vacuum to the blanket chamber holding the blanket down upon the base plate.
The invention described in the copending patent application is prone to certain operational problems of the vacuum frame. There were two types of glass breakage. One was shattering of the glass from implosion. This shattering is the result of external air pressure on the outside of the glass not being countered by a sufficient degree of air pressure on the blanket side of the glass. This "breaking force" results because the blanket is being held away from the glass by vacuum in the blanket chamber during the initial evacuation of air from the printing chamber. Therefore, the air pressure in the printing chamber is reduced below that of the external atmospheric air pressure on the outside of the glass. The result is the "uneven air pressure". This breaking force is relieved once the blanket has risen to the glass and the blanket chamber is restored to atmospheric pressure.
There is no significant "breaking force" in standard vacuum frames because the blanket, which is not held away, rises almost immediately, and the atmospheric pressure is in contact with the outside of the blanket throughout the operation of the unit. Thus, the atmospheric pressure is transmitted from the blanket to the blanket side of the glass.
The severity of the "breaking force" is dictated by the level of vacuum achieved in the blanket chamber. This is apparent since the blanket chamber vacuum level determines at what level of printing chamber vacuum the blanket begins to rise. Obviously, the blanket will rise when the printing chamber vacuum level is somewhat greater than the blanket chamber vacuum level. The force of gravity must be counter-acted to lift the blanket.
The other type of glass breakage is what will be referred to as a "pin break". The graphic arts industry makes use of metal register pins placed into the printing chamber to hold the various pieces of work in the proper position. A register pin interferes with the normal flexing of the glass which results from the "uneven air pressure". Thus, the glass tries to flex around the pin which is being pressed hard against the immovable aluminum plate. Depending upon the size and location of the pin, the glass may break at the pin location.